This artist's conception illustrates 1E 1207.4-5209, a
neutron star with a polar hot spot and a strong
magnetic field (purple lines). The neutron star is
about 7,000 light years from Earth.

The graph in the box to the right of the star shows
the expected (dashed blue line) and the observed (solid
green line) spectra of the X-radiation from the hot
spot. The dips in the observed spectrum are absorption
features due to gas in the atmosphere of the neutron
star.

According to the team of scientists that made the
Chandra observation of 1E 1207, the most likely
explanation for the dips in the spectrum is absorption
by helium ions in a magnetic field about a hundred
trillion times more intense than the Earth's magnetic
field. This interpretation implies that the strong
gravity of the neutron star has reduced the energy of
the photons by 17 percent. The reduction of photon
energy, known as the gravitational redshift, enables
astronomers to relate the mass to the radius of the
star. This information can then be used to help
determine whether the collapsed star is composed mostly
of neutrons, or contains large amounts of sub-nuclear
particles called pions, kaons, or free quarks.